Current technology relating to turbocharger systems for supercharging internal combustion engines of the diesel type, in particular for heavy-duty vehicles, which usually include a single-stage compressor that is driven by a single-stage turbine, both of radial type.
Superchargers suitable for a diesel engine with a stroke volume of 6 to 20 liters normally have an efficiency, under stationary conditions, of between 50% and 60% (?compressor*?mechanical*?turbine) In current diesel engines, the benefit of good efficiency is lower than it will be for future engines which will require higher charging pressure. Examples of systems which increase the requirement for supercharging are exhaust gas recirculation for lower emissions of nitrogen oxide or systems with variable control of inlet valves.
Turbocharger systems with an efficiency higher than 60%, under stationary conditions, afford an increased possibility of meeting future requirements for environment-friendly and economical engines. Previously, environmental requirements for diesel engines have usually led to impaired efficiency, which has consequently meant that the energy resource of the fuel has been more poorly utilized.
Modern impellers are usually provided with backswept blades where the blade angle βb2 between an imaginary extension of the center line of the blade between root section and tip section in the direction of the outlet tangent and a line which connects the center axis of the impeller to the outer tip of the blade lies below 35°.
Radial turbines used in turbochargers are often provided with scallop cutouts between the turbine blades (see FIG. 4) for reducing the mass of the turbine wheel, which results in improved transient response, that is to say increases the capacity of the turbine wheel for reacting to an increased exhaust gas flow. This makes it possible for an engine to increase speed more rapidly by virtue of the scallop cutouts reducing the polar moment of inertia because they eliminate material at the periphery of the turbine wheel. However, the scallop cutouts have a negative effect on the efficiency of the turbine owing to flow leakage from the pressure side to the suction side at the outer ends of the turbine blades. Another reason for providing the turbine with scallop cutouts is to reduce stresses owing to uneven temperature during starting, stopping and load changes. Problems of uneven temperature distribution are greater in wheels of large diameter.
A disadvantage of increasing the blade angle/3 of the compressor is that the peripheral speed and thus stresses in the impeller increase for the same pressure ratio. This means that materials with greater strength properties may be required. For example, the current cast aluminum impellers and wheels may be replaced by considerably more expensive forged and machined aluminum or titanium components.